1. Field of the Invention
The present invention relates to a solid-state image sensing device, an image reading apparatus, and an image forming apparatus.
2. Description of the Related Art
A resolution conversion technique is conventionally known which increases an image reading rate and reduces the size of read image data by reading images at a resolution lower than the resolution at which the image reading apparatus can perform reading. By reading images at a lowered resolution, the amount of light emission from a reading light source can be kept down and thereby power saving can be achieved.
In a case where the resolution in image reading is lowered to enhance the productivity in reading of a document, an image reading period (line period) in a main-scanning direction is shortened, for example. In this case, accumulation time of electric charge, which are generated by photoelectrical conversion by an image sensor to read the document, may be shortened and thereby an output level of the image sensor may decrease (i.e., an SN ratio regarding the image may be deteriorated and image quality may be degraded). For preventing the SN ratio regarding the image from being deteriorated by the resolution conversion, it is known to add successive pixel data so as to suppress the decrease in the output level of the image sensor.
Japanese Patent Application Laid-open No. 2004-048167 discloses an image reading apparatus having a mode in which a signal formed by adding two image signals is used as pixel data and a mode in which pixel signals of even-numbered light receiving elements are used as image data so as to perform read operation at a high rate in a sub-scanning direction.
However, in image sensors, dark current flows even when no light is incident. As a result, electric charge is stored as an offset. That is, when pixel signals including the offsets are added for resolution conversion, a dynamic range of image data after resolution conversion is disadvantageously narrowed.
In view of the above, there is a need to provide a solid-state image sensing device, an image reading apparatus, and an image forming apparatus which can prevent the dynamic range of image data from being narrowed even when resolution conversion is performed.